Powder coatings are polymer materials blended with other additives used to coat various substrates. Such powder coatings are today used in a variety of industries to provide protective barriers to the substrates that they coat. Powder coatings are particularly desirable today because they are environmentally friendly. They are typically applied without the use of toxic solvents, and overspray material from the application process can be reused because it has not been cured onto the substrate. To form optimum powder coatings, it is generally necessary to use polymers which have narrow molecular weight distributions. This, in turn, provides the polymer with a small polydispersity. It is also necessary to attempt to keep the polymer from curing prior to being used as a coating. In other words, it is important that minima cross-linking take place during the blending of the coating materials. Finally, it is highly desirable that the resultant powder coating produced have a narrow particle size distribution on the order of about 15 to 40 μm.
Historically, powder coatings have been made by a multiple stage process in which each stage is a unique and separate operation. First, the polymer or base resin is polymerized in a solvent based reaction. The solvent is recovered, leaving a dry resin. Typical resins used to make powder coatings may include, but are not necessarily limited to, epoxy, polyester, acrylics, and mixtures thereof.
Next, the resin is premixed, to facilitate homogeneous mixing, with a cross-linking agent and additives such as flow agents, pigments, and interfacial agents, Subsequently, the mixture is processed through a twin screw extruder at approximately 160° C. to form a homogeneous powder coating material. The material is cooled prior to grinding the material into a powder. The powder is finally separated by size into its classification.
The drawbacks to this process are polymerizing a resin with a wider molecular weight distribution and a higher polydispersity (approximately 2.6-3.0). Premature curing caused by the high heat during extrusion is also a problem. Furthermore, the extrusion process does not to produce a fully homogeneous dispersion. The high temperature of the extrusion process forces the manufacturer to use high temperature compatible cross-linking agents . Yet another drawback is that grinding produces dust and material losses and consumes energy, and creates a wider particle size distribution. Also, the process requires multiple stages that require large amounts of floor space and are dedicated to operating at a particular batch size.
Wider molecular weight distribution is undesired because the resin is less effective. For a particular application, a target molecular weight for the resin is optimal for that application. As the molecular weight diverges from the target molecular weight, the properties of the resin change and become less optimal. Staying narrowly close to the target molecular weight achieves the desired optimum properties for the resin. Molecular weight of the resin affects adhesion to the substrate and porosity of the coating which, in turn, affects the protection of the substrate. The molecular weight distribution is measured as polydispersity. Polydispersity is the weight average molecular weight divided by the number average molecular weight. As the polydispersity number decreases, the molecular weight distribution becomes narrower and closer to the average molecular weight of the resin. Historically, polydispersity has ranged from 2.6 to 3.0 for resins. Preferably, polydispersity is desired to be about 2 or less.
Analogous to molecular weight is particle size. A targeted particle size is optimal for a given application. As the particle size diverges from the target molecular weight, the properties of the resin change and become less optimal. Particle size affects the porosity of the coating and the ability to protect the substrate and the adhesion of the resin to the substrate.
The selection of cross-linking agents is limited by the high temperatures required for melt blending in an extruder. Cross-linking agents have to be selected that cure at a temperature above the operating temperature of the extruder to prevent premature curing of the cross-inking agent. The premature curing causes the resin to cross-link with itself and reduces the available cross-linking sites that would be available to cross-link with the substrate. This makes for a weaker coating and also reduces the gloss of the resin. Also, the high temperature cross-linking agent requires that high temperatures be used when curing the resin to the substrate. This adds additional energy costs. Lower blending temperatures would permit a larger number of cross-linking agents to be selected based on functionality and costs of the cross-linking agent, and lowers energy costs to cure the resin to the substrate.
Additionally, the extrusion process does not provide a uniform blending. This is because there will be zones of varying temperature which affect the melt of the resin and the fluidity of the melt. Uniform blending is desired to reduce the variation in the powder coating and increase the effectiveness of the additives in the powder coating and the powder coating itself.
The traditional powder coating production process comprises many dissimilar process operations. The first step is polymerizing the resin in a reactor. Once reacted, the resin is separated from its solvent and prepared for extrusion blending. Next, the resin is blended with the additives to make the powder coating material. After blending, the powder coating is processed through a grinder to form particles of a desired size, and finally sorted according to the desired size. Each of these steps is unique and requires different processing equipment. This increases the capital cost of purchasing these various types of equipment and the amount of floor space needed to store the equipment.
Additionally, these pieces of equipment are designed to operate with a predetermined batch size. Particularly, the extruder has a minimum amount of material required to reach a steady state operation. If a small batch of a particular powder coating product is desired, the amount of waste in proportion to the amount of product generated is high as compared to a large batch run. Alternatively, a large batch could be produced; however, storage costs would be incurred storing the excess, and if stored for too long, the quality of the product deteriorates.
Similarly, because of the dissimilarity between the processing steps, material has to be produced and stored from each step before being processed by the next step so that there is a surplus amount of material to continuously feed the process. This adds additional costs to store this intermediate material in terms of warehouse space and environmental controls to maintain the quality of the material.
The use of supercritical fluids as a solvent in various steps of this prior art process is well known. For example, U.S. Pat. No. 5,328,972 to Dada et al. discloses forming a reaction mixture of one or more polymerizable monomers, and a free radical initiator in supercritical carbon dioxide at an elevated temperature of at least 200° C. and an elevated pressure of at least 3,500 psi such that the monomer is present in the reaction mixture at a level below 20% by weight of the supercritical carbon dioxide. This patent also discloses that the process produces polymer products having high purity, molecular weights below 5,000, and polydispersity below 2.5 without resorting to techniques such as chain transfer agents or chain stopping agents.
U.S. Pat No. 4,703,105 to Allada discloses bringing polymerizates into contact with selected gases in near-critical to supercritical states to inhibit deep polymerization and decomposition while substantially improving residue extraction. It is disclosed that this process enhances the molecular weight distribution of the polymers and results in product containing substantially reduced amounts of low molecular weight components and provides a relatively narrow molecular weight distribution product.
U.S. Pat. Nos. 5,118,530 and 5,063,267 to Hanneman et al. are directed to hydrogen silsesquioxane resin fraction derived from an extraction process using one or more fluids at, near, or above their critical state. The fractions comprise narrow molecular weight fractions with a polydispersity less than about 3.0 or fractions useful for applying coatings on substrates. The process can extract various narrow molecular weight fractions because the sensitivity of the fluids to both temperature and pressure changes allow for accurate control of solvent strength. Once the desired molecular weight fraction has been dissolved in the solvent, it is passed through an area in which the temperature or pressure is changed, such that the fraction is no longer soluble in the extraction fluid, and therefore, precipitates out of solution. The resins are fractionated using a variety of fluids at, near or above their critical point.
U.S. Pat. Nos. 5,252,620 and 5,128,382 to Elliott, Jr. et al. are directed to the preparation of organic microcellular foams prepared by polymerizing directly in a near critical fluid and pursuing the supercritical drying in the same reactor. A critical variable in the choice of a dilutent is identified as having a strong enough solvent power to stabilize the power matrix, but a low enough critical temperature to permit critical point drying without damage to the polymer matrix.
U.S. Pat. No. 5,212,229 to Taylor et al. is directed to a coating composition comprising monodispersed acrylic polymer solutions containing supercritical, near-supercritical, or sub-critical fluids as dilutent having a molecular weight that is useful as a polymeric coating vehicle. The process produces acrylic thermoplastic polymers having a molecular weight suitable for use as a coating and a polydispersity of 1 to about 1.5.
U.S. Pat. No. 5,290,827 to Shine discloses the formation of homogeneous polymer blends from thermodynamically immiscible polymers, by dissolving the components in a supercritical fluid and expanding the solution through a fine nozzle.
U.S. Pat. No. 5,412,027 to Shine also discloses the formation of homogeneous polymer blends from thermodynamically immiscible polymers, but includes block or graft copolymers in the group of thermodynamically immiscible polymers. The materials are dissolved on a supercritical fluid and expanded through a fine nozzle.
U.S. Pat. No. 5,487,965 to Odell, discusses a process for preparing a liquid developer composition by dispersing a suspension of polymer resin, colorant, charge director, and hydrocarbon carrier in a supercritical fluid to obtain finely divided colored polymeric particles. Techniques for the compatibilization of the resins or other materials through the use of a supercritical fluid was not disclosed.
U.S. Pat. No. 5,178,325 to Nielsen discloses methods for coating substrates with a coating material and a supercritical fluid. Coating formulations typically include a solids fraction containing at least one component which is capable of forming a coating on a substrate, such as a polymer component (thermoplastic or thermosetting material as well as cross linkable forming systems). A solvent fraction is also employed in order to act as a vehicle in which the solid fraction is transported from one medium to another. A coupling solvent or active solvent may additionally be utilized. The liquid mixture of polymers, a solvent component containing at least one supercritical fluid and optionally active solvent is sprayed onto a substrate to form a liquid coating.
U.S. Pat. No. 5,264,536 to Radosz discloses a supercritical separation process for polymers utilizing mixed solvents. The process can be used to remove light end or heavy end, or generate bulk fractions of low polydispersity. Adding the polymer to the solvent converts the solvent solution to a two phase mixture. The solvent utilized is a multi-component solvent including an anti-solvent being a low capacity component, and a high capacity component. Carbon dioxide is exemplified as an anti-solvent. The polymer to be fractionated is to be contacted with the solvent having two components and a temperature and pressure are chosen so as to determine a different average molecular weight for each fraction according to the selectivity and capacity of the solvent for the desired polymer fractions. Each phase is then separated, and the polymer fractions are separated from the solvent.
U.S. Pat. Nos. 4,582,731, 4,734,227, and 4,734,451 to Smith are directed to the deposition of solid films or the formation of fine powders by dissolving a solid material into a supercritical fluid solution at an elevated pressure, and then expanding rapidly through a heated nozzle into a region of relatively low pressure.
With regard to 4,734,451 patent, thin films are deposited and fine powders are formed utilizing a supercritical fluid injection molecular spray. Individual molecules of very small clusters of the solute are produced, which may then be deposited as a film on any given substrate, or by promoting molecular nucleation or clustering, as fine powder. The primary requirement for fluid injection molecular spray is that materials to be deposited on a suitable precursor must be soluble in the supercritical fluid.
In the 4,734,227 patent, a secondary solvent mutually soluble with the solute and primary solvent and having a higher critical temperature than that of the primary solvent is used in a low concentration to maintain the solute in a transient liquid state. The solute is discharged in the form of long, thin fibers, which are collected at distance sufficient to allow them to solidify in the low pressure region. The process modifies rapid expansion of a supercritical solution containing a polymer solute and an appropriate supercritical solvent so that the solute passes briefly through an intermediate phase, rather than directly to a solid from the solution.
U.S. Pat. Nos. 4,313,737 and 4,364,740 both to Massey et al. disclose a process for treating a hydrocarbonaceous material, such as coal, to separate the solid into a hydrocarbonaceous enriched fraction and a mineral and sulfur enriched fraction. The material is admixed with a low molecular weight alcohol, and the resulting slurry is then heated and pressurized to a supercritical temperature and pressure. Thereafter, the slurry is subjected to a flash expansion to produce selective precipitation and explosive comminution of the components. The product resulting is an admixture of discrete hydrocarbonaceous particles and discrete mineral particles.
U.S. Pat. No. 5,126,058 to Beckman discloses a method for selectively separating commingled materials of different densities by selective density flotation of the materials in a fluid in the vicinity of its critical point. The process is applicable to polymeric waste commingled with other materials, such as wood, paper, metals and glass. It also discuses synthetic polymer waste streams, such as those composed of high and low density polyethylene, polypropylene, polystyrene in foamed and bulk form, polyethyleneterephthalate)PET and (polyvinyl chloride)PVC. A method is disclosed for selectively separating a component material from a mixture of commingled materials of different densities by selective density floatation, comprising the steps of introducing the mixture of commingled materials into a vessel, introducing a fluid into the vessel, the fluid having a range of densities in the vicinity of its critical point such that said fluids density may be set to be between the density of one component of the commingled components and the densities of the remaining components, adjusting the temperature and pressure of the fluid to set the density to selectively float the portion of the materials having a density less than the set density o the fluid, and separating the component which has been selectively divided. The fluid must exhibit the proper density range in the vicinity of its critical point, and should be a poor solvent for the polymers and other materials to be separated.
The art has not addressed the problems, however, of reduction of processing steps, elimination of the grinding stage which produces dust and material loss, compatibility between process stages, having small batch capability of blending powder coating ingredients in existing production equipment, utilization of low temperature cross-linking agents, fusing, curing, achieving increased homogeneous blending, encapsulation as the blending method, achieving a narrow particle size distribution of the powder coating, and producing a powder coating with increased gloss and strength.